Section Assessment

Section Summary

I Classification of igneous rocks is based on three main characteristics.

I The rate of cooling determines crystal size.

I Ores often occur in pegmatites. Diamonds occur in kimberlites.

I Some igneous rocks are used as building materials because of their strength, durability, and beauty.

Understand Main Ideas

1. i man HTM Infer why obsidian, which is black or red in color, can have a granitic composition.

2. Describe the three major compositional groups of igneous rocks.

3. Apply what you know about cooling rates to explain differences in crystal sizes.

4. Distinguish between andesite and diorite using two physical properties of igneous rocks.

Think Critically

5. Speculate why there are almost no extrusive ultrabasic rocks in Earth's crust.

6. Determine whether quartz or plagioclase feldspar is more likely to form a well-shaped crystal in an igneous rock. Explain.

■Miiii^ Earth Science

7. A granite slab has a density of 2.7 g/cm3. What is the mass of a 2-cm-thick countertop that is 0.6 m x 2.5 m? How many grams is this?

Section 2 • Classification of Igneous Rocks 123

(t)Koos van der Lende/age fotoStock, (b)Ken Lucas/Visuals Unlimited

Section 2 • Classification of Igneous Rocks 123

(t)Koos van der Lende/age fotoStock, (b)Ken Lucas/Visuals Unlimited

Moon Rocks

During each of the six Apollo missions, lunar rocks were collected with the hope of providing information about the Moon's origin, history, and environment. How do moon rocks compare with rocks on Earth?

Moon rock types Between 1969 and 1972, astronauts collected approximately 380 kg of lunar rocks. The 2415 individual pieces range in size from a grain of sand to a basketball.

Generally, moon rocks vary in color from gray to black to white to green. Some rocks are glassy, some are hard, and others are fragile. Analysis of the rocks has revealed at least three different rock types on the Moon. Basaltic rocks formed from lava flows and volcanic ash that reached the surface through cracks and fissures caused by meteorite impacts. Breccias formed when meteorites shattered rocks and then fused the pieces together with the heat generated by the impact. Pristine rock is rock that has not been hit by meteorites. Pristine rock is commonly composed of calcium-rich plagioclase feldspar and is gray in color.

Moon rock composition Moon rocks are unique in two ways. First, they contain no water and are not oxidized. Considering how much iron is contained in the rocks, this is a sharp contrast to weathered and rusty iron-bearing rocks on Earth. Second, the surfaces of some moon rocks are covered with tiny pock-marks called zap pits. These are caused by micrometeoroids that impact the rocks on the Moon's surface. Zap pits do not occur on Earth rocks because friction from Earth's atmosphere causes tiny meteoroids to burn up long before they reach Earth's surface.

Moon rock classification Scientists use the same categories for classifying lunar rocks as they use for igneous rocks on Earth.

Sea Basalts Moon
This scientist is studying a piece of basalt that was collected from the lunar surface during the Apollo 15 mission.

Based on mineral composition, scientists named a new class of moon rocks called KREEP rocks. These contain high amounts of potassium (K), rare Earth elements (REE), and phosphorus (P). These rocks are more radioactive and higher in thorium than Earth rocks.

Moon rock research Lunar rock research continues at the Johnson Space Center in Houston, Texas. The rocks are protected in stainless steel vaults in a dry nitrogen atmosphere to keep them moisture- and rust-free. Scientists continue to pose questions about these rocks as they study the Moon's origin and history.

C^ESZ>Earth Science

Lunar Rock Game Use resources to design a game that involves the collection and analysis of lunar rocks by scientists. Trade games with classmates to increase your understanding of lunar rocks.

124 Chapter 5 • Igneous Rocks

Roger Ressmeyer/CORBIS


Background: The rate at which magma cools affects the grain size of the resulting igneous rock. Observing the crystallization of magma is difficult because molten rock is very hot and the crystallization process is sometimes very slow. Other materials, however, crystallize at lower temperatures. These materials can be used to model crystal formation.

Question: How do minerals crystallize from magma? Materials clean, plastic petri dishes thermometer saturated alum solution paper towels

200-mL glass beaker water magnifying lens hot plate dark-colored construction paper

WARNING: The alum solution can cause skin irritation and will be hot when it is first poured into the petri dishes. If splattering occurs, wash skin with cold water.


1. Read and complete the lab safety form.

2. As a group, plan how you will change the cooling rate of a hot solution poured into a petri dish. Each group member should choose a petri dish in a predetermined location to observe during the investigation. Make sure your teacher approves your plan before you begin.

3. Place a piece of dark-colored construction paper on a level surface where it will not be disturbed. Be sure to put the paper in all of the predetermined locations. Place the petri dishes on top of the paper.

4. Using the glass beaker, obtain about 150 mL of saturated alum solution from your teacher. The temperature should be about 95°C to 98°C, just below boiling temperature.

5. Carefully pour some of the solution into each petri dish so that it is half full. Use caution when pouring the hot liquid to avoid splatters and burns.

Someone Making Crystals Petri Dish

6. Every 5 min for 30 min, record your observations of your petri dish. Make drawings of any crystals that begin to form.

Analyze and Conclude

1. Compare your methods of cooling with those of other groups. Did some methods appear to work better than others? Explain.

2. Examine your alum crystals. What do the crystals look like? Are they all the same size? Do all the crystals have the same shape?

3. Draw the most common crystal shape in your science journal. Compare your drawings with those of other groups. Describe any patterns that you see.

4. Deduce what factors affected the size of the crystals in the different petri dishes. How do you know?

5. Infer why the crystals changed shape as they grew.

6. Compare and contrast this experiment with magma crystallization.

7. Evaluate the relationship between cooling rate and crystal formation.

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  • johnny white
    Why there are almost no extrusive ultrabasic rocks in earth's crust?
    8 years ago
  • aziz russom
    What si the mass of a 2cm thick countertop that is 0.6 m by 2.5 m?
    22 days ago

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